Deployable Defences

[Monk]

Alrighty, I have a few questions about deployable defences, and I seek your opinions on their usefullness.
Firstly, what are all the types of deployable defences, and what do they all do? I know that chevaux-de-frises/spiky things stop cavalry, but how effective are they in disrupting a bayonet charge? And how effective are the giant sandbag things in a firefight? And are the cannon sandbags meant to be anti-melee or gunfire?

Secondly, can you utilise these defences at any stage of the game, or do some need to be researched? Are there any factors affecting whether you can deploy defences (apart from the obvious one that you have to be defending)?

Finally, a miscellaneous inquiry-can all light infantry lay stakes, and can they lay more than one set of stakes throughout a battle? If so, is there a limit on this?

Thanks!

Infantry can climb over the anti-cav defenses. It would sorta be overpowered if it stopped infantry and cavalry both, you'd win every battle without trying.

The only other type I can think of that you've not mentioned is earthen trenches, which give your troops cover.